Lithium secondary batteries, which are lightweight and have high energy density, are mainly commercialized as the power source for portable devices. Also, lithium secondary batteries are currently receiving attention as large-sized, high-output power sources (e.g., power sources for automobiles). They are being actively developed.
Lithium secondary batteries have an insulating layer between the positive electrode and the negative electrode. The insulating layer has the function of electrically insulating the electrodes from each other while retaining an electrolyte. Since the insulating layer easily shrinks, the positive electrode and the negative electrode tend to come into physical contact with each other to cause an internal short-circuit when the lithium secondary battery is left in a very high temperature environment for an extended period of time. Recently, the capacity of lithium secondary batteries is increasingly becoming higher, and thus, the insulating layer is increasing becoming thinner. Due particularly to this trend, an internal short-circuit is becoming an increasingly important problem to be solved. Once an internal short-circuit occurs, the short-circuit further expands due to Joule's heat generated by the short-circuit current. In some cases, the battery may overheat.
When an internal short-circuit occurs in a battery, it is very important to assure safety. Hence, techniques to enhance battery safety under an internal short-circuit condition have been actively developed. For example, Patent Document 1 proposes a technique in which insulating tape is affixed to the exposed part of a positive or negative electrode current collector to prevent an internal short-circuit between the current collectors. Also, Patent Document 2 proposes a technique in which an ion-conductive insulating layer composed of ceramic particles and a binder is printed on an electrode plate.
Further, to assure safety under an internal short-circuit condition, it is also very important to accurately evaluate the safety of a battery under an internal short-circuit condition. Battery evaluation tests for evaluating exothermic behavior under an internal short-circuit condition as a measure of the safety of batteries such as lithium secondary batteries are defined, for example, by UL standards for lithium batteries (UL1642) and guideline of Battery Association of Japan (SBA G1101-1997 Guideline for Safety Evaluation on Lithium Secondary Cells)(See Patent Document 3).
Examples of such evaluation tests include a nail penetration test and a crush test. A nail penetration test is performed by sticking a nail into a side face of a battery to cause a short-circuit between the positive electrode, the negative electrode, and the nail inside the battery, and observing changes in battery temperature, battery voltage, etc. caused by Joule's heat generated by the short-circuit current flowing through the short-circuit. Also, a crush test is performed by physically deforming a battery by using a round bar, square bar, flat plate, etc. to cause an internal short-circuit between the positive electrode and the negative electrode, and observing changes in battery temperature, battery voltage, etc.    Patent Document 1: Japanese Laid-Open Patent Publication No. 2004-247064    Patent Document 2: Japanese Laid-Open Patent Publication No. Hei 10-106530    Patent Document 3: Japanese Laid-Open Patent Publication No. Hei 11-102729